This application is for an NIDDK Multi-Center Clinical Study Implementation Planning Grant (U34) to plan a clinical trial on the efficacy of lifestyle intervention to treat obese men with erectile dysfunction (ED). We have identified the 2 clinical centers and coordinating center and a team of well-established experts in ED, lifestyle intervention, recruitment and retention of large cohorts and coordinating multi-center trials. The U-34 planning phase will allow us to complete the scientific planning and administrative activities that are required to prepare to recruit subjects for this trial. ED, defined as the consistent inability to achieve and/or maintain an erection sufficient for sexual performance, is a significant public health concern because of its prevalence among middle aged men, its strong association with diminished quality of life, and its recently-established link to endothelial dysfunction and likely role as a harbinger of cardiovascular disease (CVD). Recent studies suggest that that lifestyle intervention (weight loss and increased activity) may offer a promising new approach that could improve ED as well as reducing CVD risk. A definitive randomized trial is needed to determine whether lifestyle intervention is effective for ED and to identify those men who are most likely to respond to lifestyle change. We propose a multi-center randomized controlled trial evaluating lifestyle intervention (weight loss and physical activity) to treat obese men with ED. This project, entitled Lifestyle Intervention to Treat Erectile Function (LITE), will include 300 obese men with ED from two clinical centers who will be randomly assigned to an intensive lifestyle intervention (based on protocols used successfully in several recent NIDDK trials) or delayed treatment control and followed at 6 and 12 months post-randomization. The primary goal is to determine whether randomization to an intensive lifestyle intervention results in greater improvement in ED compared to the control condition. We will also compare the lifestyle intervention and control conditions on normalization of ED, cardio-metabolic risk factors, lower urinary tract symptoms (LUTS), and patient well-being at 12-months and identify baseline characteristics that moderate the effect of treatment, potentially identifying men most likely to experience improved or normalized erectile function. Blood samples will be collected to assess biomarkers of oxidative damage, endothelial function and inflammation to investigate the mechanisms by which lifestyle intervention improves ED . Since ED may be a harbinger of CVD, lifestyle interventions may be of particular importance because they may not only be used as a first-line treatment for ED, but also may prevent or treat the many manifestations of CVD and thus would have a significant impact on personal and public health.